The use of substantially linear hydroxypolyurethanes as raw materials for gluing various substances, such as plasticizer-containing PVC or natural or synthetic rubbers, either to surfaces of the same material or to other materials has been described in German Auslegeschrift No. 1,256,822.
One disadvantage of these adhesives is that they do not adhere sufficiently firmly to certain types of rubber, particularly to soft, transparent rubber. Furthermore, this defect may only be improved by an additional, rather complicated operation. Among the numerous pre-treatments of rubber surfaces which have been described in the literature may be mentioned the painting of the surface with oxidizing acids, such as sulfuric acid (German Pat. No. 807,719), halogenation using compounds which split off chlorine or bromine (German Offenlegungsschrift No. 2,550,823) and application of a pre-coat of polychloroprene or other halogen-containing polymer (U.S. Pat. No. 3,917,742). All these methods involve an additional process step and therefore increase the cost. They may also give rise to toxic products, e.g. in the case of the pre-treatment with sulfuric acid or halogens, which require elaborate protective measures.
The improvement in the adherence of polyurethanes to various materials by the incorporation of ions (especially cations) has been repeatedly described (e.g. D. Dieterich, W. Keberle and H. Witt, Ang. Chemie 82, 53-63 (1970)). U.S. Pat. No. 3,658,939 discloses the improvement obtained in the adherence of polyurethanes to metal surfaces when the polyurethane applied from solution contains free carboxyl groups which have been chemically fixed by way of 2,2-dimethylol-propionic acid.
However, the adherence of such polyurethanes to various rubber materials is not sufficiently strong. Moreover, the inadequate resistance to separation of rubber materials which have been glued using such modified polyurethanes cannot be improved by simply increasing the ionic or carboxyl groups in the polyurethane.
Indications as to how the disadvantages described above may be obviated are found in German Auslegeschrift No. 2,113,631 which teaches the addition of carboxylic acids, preferably fumaric acid, to commercial polyurethane adhesive solutions, whereby a considerable improvement in adherence to rubber materials is obtained without any elaborate pre-treatment. One disadvantage of this method, however, is the powerful hydrolytic action of the added substances on the basic polyurethane which considerably reduces both the stability of the adhesives in storage and the strength of the bonds, particularly at elevated temperatures in a moist climate.
It has now surprisingly been found that dihydroxypolyurethanes which contain both chemically fixed ionic groups and chemically fixed carboxyl groups and which are prepared by the process according to the invention described below are eminently suitable for gluing rubber. The obtainable resistances to separation of the bonds are substantially higher than those obtained using polyurethanes known in the art. Additionally, the disadvantages mentioned above, which are observed when carboxylic acids are added to solutions of polyurethane adhesives, no longer occur. The novel products obtained by the process according to the present invention are suitable for bonding a variety of different substrates.